A Requiem for Comet ISON?


By Jerry Granok

Special to The SUN

Comet C/2012 S1 (ISON) was born at the beginning of our solar system, over 4.5 billion years ago.

An agglomeration of dust, rock and ices, it never became part of a planet. Something, perhaps a passing star or a collision with other space material, happened to nudge it from its resting place billions of miles away from the sun in the Oort Cloud towards the sun.

Slowly at first, but constantly gathering speed, it drifted inwards towards the sun. After untold years, Comet C/2012 S1 (ISON) was discovered on Sept. 21, 2012, by astronomers Artyom Novichonok and Vitali Nevski, using a 16-inch telescope that is part of the International Scientific Optical Network (ISON), and after which the comet was named.

Once it had been discovered, excitement grew as its orbit was calculated and determined that it was a new comet that had never entered the inner solar system, and was plotted to pass very close to the sun — a “sungrazer.”

There was even more excitement when the comet appeared to be brighter than typical at the distances it was being observed. This led to speculation that it would become a very bright comet as it approached the sun, possibly even being visible in daytime. Some folks got caught up in the hyperbole and predicted it would be as bright as the full moon. It would be the “Comet of the Century.”

Someone once said that comets are like cats: they both have tails and you can’t predict what they are going to do. Comet ISON did just that.

The only thing fairly certain was its orbit. After its initial brightening, it slowed its rate of brightening and started behaving like a more typical comet. Astronomers started taking pictures of the comet as it grew a noticeable tail. There was a brief uptick in the comet’s brightness in mid-November, just two weeks before it was to experience its closest approach to the sun (perihelion) on Nov. 28, 2013, and just 700,000 miles above the 10,000 degree solar surface. When we think something has very little chance of succeeding we describe it as “a snowball’s chance in hell.” ISON was a snowball in hell. Scientists couldn’t be sure if ISON would survive the intense environment at perihelion but, because it was several miles in diameter and would pass through the most intense part of the solar environment in a few hours, they thought something would survive.

The best time to observe Comet ISON from Pagosa Springs in November was between 5 and 5:30 a.m. when the comet was highest in the sky and the morning twilight was just beginning. This favored early risers and nightowls who stayed up until dawn. Unfortunately, this was also the coldest part of the night and the comet was never very high in the sky. At best, the comet was only 30 degrees above the eastern horizon which meant an observer was looking through twice as much atmosphere as when observing something directly overhead. When it would no longer clear the mountains to the east, an observer would be looking through six to seven times as much atmosphere. All this, coupled with growing morning twilight, makes an already dim object like a comet that much harder to see.

The San Juan Stargazers had wanted to hold public comet observing sessions, but decided against it because of the poor viewing conditions. It was hoped that December would offer better opportunities for comet viewing as the still-bright comet rose earlier in the morning sky.

As the comet approached its Nov. 28 perihelion, it came too close to the sun in the sky to safely view by eye. Fortunately, a flotilla of solar-observing satellites with names like SOHO (http://sohowww.nascom.nasa.gov/), SDO (http://sdo.gsfc.nasa.gov/), and Stereo A and B (http://stereo.gsfc.nasa.gov/) were in place in space to safely view the comet from multiple angles. The satellites carried a mix of optical instruments for viewing the sun. Some of the instruments used an occulting disk to block out the direct image of the face of the sun so that fainter objects of interest in the vicinity of the sun could be viewed. Other instruments were designed to safely see the full face of the sun at multiple wavelengths of light. It was planned that the multiple instruments would be able to follow Comet ISON as it approached the sun, went through perihelion, and then receded from the sun. By Dec. 6, the comet would be far enough away from the sun that other telescopes could again be used to view and photograph the comet as it returned to outer space.

By the last few days before perihelion, the comet finally was visible in the field of view of some of the instruments. There it was: it grew brighter by the hour and there was no denying it had a dramatic tail. Just before the comet approached the area hidden by the occulting disk, its brightness had surpassed that of the star Arcturus that was in the same field of view and caused a “blooming” of its image due to oversaturation of the pixels in the imaging system. For two hours around the expected time of perihelion, NASA was running a live Google plus feed available on YouTube with live commentary from cometary experts.

Then it happened. With excitement at a fever pitch, the comet seemed to lose its head, the coma — the round cloud of dust and gases that surrounds the comet body — and became a very bright, arrow-shaped, headless tail just as it disappeared behind the occulting disk. It was expected that the SDO satellite would see something of the comet as it passed through the sun’s outer atmosphere just as it had with Comet Lovejoy in 2011 (www.nasa.gov/mission_pages/sunearth/news/comet-lovejoy.html), but nothing was seen. The frantic commentators declared the comet dead and, no sooner had they signed off, something reappeared from behind the occulting disk on the comet’s projected trajectory.

Was the comet still alive?

Like a cat, had it just used up one of its lives?

Hope that the comet had survived and would become a naked eye object again rapidly faded over the next few hours though, as what appeared to be a stretched-out comet evolved into a rapidly dispersing cloud of dust. As of this writing, it is believed the comet is really dead after all, but the final decision will not be made until after Dec. 6 when non-solar telescopes can safely view the remaining dust cloud. The discussion can be followed on the NASA Comet ISON Observing Campaign website (http://www.isoncampaign.org/).

Comet ISON may not have turned out to be the comet of the century, but don’t despair. There are other comets out there right now that can be seen in the early morning sky, some with optical aids and some with the naked eye if you know where to look and what to look for. Check the Comet Chasing website (http://cometchasing.skyhound.com/) for information about what is out there and where to find it. And, coming in late 2014, is another new comet from the Oort Cloud, Comet C/2013 A1 (Siding Springs), that is expected to pass Mars at a distance that is less than one-half of the Earth-Moon distance.

The San Juan Stargazers astronomy club will hold its regular monthly meeting on Thursday, Dec. 19, starting at 7 p.m. in the conference room at the Chamber of Commerce office on Hot Springs Boulevard. The topic of discussion will be comets, Comet ISON and what may have happened to it. By meeting time, initial telescopic examinations of the dust cloud should have been reported and we will know if Comet ISON is really dead or if, like a cat, it has more lives (and surprises) left.

The San Juan Stargazers is open to anyone with an interest in astronomy, regardless of age or experience. Club meetings are held the third Thursday of the month in the conference room at the Chamber of Commerce office. The club has a website, www.sanjuanstargazers.com, as well as an e-mail address, sjstargazers@gmail.com, and a phone number, (970) 335-8286, to help communicate with the public.